Their was a group called ACTIV - Army Concept Team In Vietnam that tested and evaluated many things. One of those reports was on the XM174 automatic grenade launcher. It fired low velocity grenades like the M79 and M203. Ultimately it wasn't adopted but I'm looking for their final report. It was tested and used for a time by several Marine units in the field and by USAF base security during the war. 

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I have a copy of that report in my library.  It's a hard copy printed from old microfiche so the images are bad but the text is fairly legible.  Any part in particular you need?

Ordnance - Awesome. Just interested in the field reports when it was used on Marine patrols. 

Unfortunately, I can't help with that.  The ACTIV report on the XM174 was dated November, 1968.  For field testing, only 10 weapons were issued to units of the 4th and 25th Infantry Divisions and 5th Special Forces Group between May and September, 1968.  None were tested by Marine Corps or Air Force units at that time unless it was under a separate program.

 

The Army test troops generally liked it for base or vehicular use but not for use in the field, mainly because of the weight and awkwardness of sling carry.  With a loaded 12-rd magazine the XM174 was 24 pounds and 45 pounds if the tripod was added.  As you can see in the document attached, it was recommended for adoption.  I don't know how many Aerojet ultimately made for the Government but several hundred at least.  The photos I have of USMC or USAF use in Vietnam all seem to be from 1969 to 1971 or later so I assume distribution came after or at the same time as Army issue.  They could have been guns shared by the Army or perhaps contracted directly by the other services but I have no evidence either way.

 

I'll add a few photos including my all time favorite U.S. Military tech manual image from the pocket TM.  Yes, that photo of the woman holding the XM174 is actually in the manual.

Thanks! Great information. There are a few references in the Texas Tech Vietnam archives from the USMC. A few patrol AARs list it as being used on patrols, but nothing specific as to its effectiveness other than one I saw where it was fired against enemy troops. Another document is from a Marine service Bn being responsible for maintenance for it. Interesting system.

Just found this material when I searched for the TM you posted. 

PDF Doc from Army Weapons Command

I just ran across your posting.  I saw one in use by Air Force Security Police in late 1971 at Cam Ranh Bay.  As I remember it was not mounted but hand carried, drum fed.  I saw them empty the drum, holding in the maximum evaluated position they emptied the drum before the first round hit the ground.  Impressive display, to bad they halted development.